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Measuring Blow by, The Real Test For Internal Engine Condition

Blow by Measurement

fig1Blow by is generally recorded in liters per minute, but a water manometer may be used to measure blow by from the breather tube after fabricating the following adaptation:

  1. Plug the end of the straight portion of a pipe tee
  2. Drill an orifice in the plug (refer to the Blow by Conversion Chart below for the appropriate orifice size)
  3. Connect the open straight portion of the pipe tee to the breather tube
  4. Connect a water manometer to the 90 degree outlet
  5. Use the Blow by Conversion Chart to convert the manometer reading to liters/minute
Blow by Conversion Chart (5.613 .mm [0.221 in] Orifice)
Inches of WaterLiters/MinuteInches of WaterLiters/Minute

Engine Testing – General Information

The engine test is a combination of an engine run-in and a performance check. The engine run-in procedure provides an operating period that allows the engine parts to achieve a final finish and fit. The performance check provides an opportunity to perform final adjustments needed to optimize tho engine performance. An engine test can be performed using either an engine dynamometer or a chassis dynamometer, If a dynamometer is not available, an engine test must be performed in a manner that simulates a dynamometer test.
Check the dynamometer before beginning the test. The dynamometer must have the capability to test the performance of the engine when the engine is operating at the maximum RPM and horsepower range (full power).
The engine crankcase pressure, often referred to as engine blowby, is an important factor that indicates when the piston rings have achieved the correct finish and fit. Rapid changes of blowby or values that exceed specifications more than 50 percent indicate that something is wrong. The engine test must be discontinued until the cause has been determined and corrected.

General Engine Test Specifications

Maintain the following limits during a chassis dynamometer test:

Intake Restriction (Maximum)
Clean Filter(light duty)254 mm H2O [10 in. H2O]
(medium duty)305 mm H2O [12 in. H20]
(heavy duty)381 mm H2O [15 in. H2O]
Dirty Filter(light duty)635 nm [25 in]
(medium duty)635 nm [25 in]
(heavy duty)635 nm [25 in]
Exhaust Back Pressure (maximum)
Industrial76 mm Hg [3.0 in.Hg]
EPA Certified144mm Hg [4.5 in. Hg]
Oxidation Catalyst152mm Hg [6.0 in. Hg]

Blow by** (at Given Speed, 100% Load)

EngineNew (L/Min.)Worn (L/Min)
4B @ 22001836
4B @ 25002040
4B @ 28002346
4BT/4BTA/B3.9 @ 22004590
4BT/4BTA/B3.9 @ 250051102
4BT/4BTA/B3.9 @ 280057114
6B @ 22002652
6B @ 25003060
6B @ 28003468
6BT/6BTA/B5.9 @ 220063126
6BT/6BTA/B5.9 @ 250076152
6BT/6BTA/B5.9 @ 280085170
**Blow by Checking tool, Part No. 3822476, has a special 5.613 mm [0.221 in.] orifice that bust be used to get an accurate reading.

Oil Pressure

Low Idle (Minimum Allowable)69 KPa [10 psi]
Rated Speed (Minimum Allowable)207 KPa [30 psi]
* Fuel Filter Restriction (Maximum)
Dirty Filter35 Kpa [30 psi]
Fuel Filter Restriction (Maximum)518 mm Hg [20.4 in hg]


NOTE: Due to variations in ratings of different engine models, refer to the specific engine data sheet for the particular engine model being tested.